The invention specifically relates to a flame retardant containing a phosphazene ring structure, a preparation method and a supercapacitor electrolyte solution using the flame retardant. The preparation method includes: under nitrogen protection, and under the action of tetrahydrofuran and anhydrous potassium carbonate, reacting hexa-substituted cyclotriphosphazene with 2, 2'-biphenol to obtain an intermediate I; reacting 4-hydroxy aniline with salicylaldehyde to obtain an intermediate II, reacting the intermediate II with paraformaldehyde to obtain an intermediate III; and finally reacting the intermediate I with the intermediate III to obtain the flame retardant containing the phosphazene ring structure shown as formula I. The flame retardant can be used as an additive of a supercapacitor electrolyte solution, so that the supercapacitor electrolyte solution can self-extinguish and reach a flame retardant effect, the use performance of the electrolyte solution is not affected, and the safety of the electrolyte solution is improved.